One feature which I think wasn't pointed out in the chapter is that each subject's auditory hallucination is of a relative of the same gender as themselves.

Assuming the same was true in ancient times, it seems strange that in the Abrahamic religions God is always male - there is no mention of a female Goddess (though the Virgin Mary almost has this status in Catholicism!) Perhaps due to their lower social status all females' hallucinations were ignored, though this seems unlikely; and presumably the Bible/Torah/Koran do report occasions when God spoke to females (presumably in a male voice).

Secondly, if the hallucinations are always of a relative, this might partly explain why (in Christianity at least) God is seen as 'the father' - perhaps the hallucination was usually of the subject's own father (at least, if the subject was male).

Finally, it's not clear why people (including the quadriplegics studied) would interpret the voice of a dead relative as being God's rather than the ghost of that relative. Though perhaps this explains the belief in dead spirits in some other religions.

That's a very interesting observation. I think you are probably correct in that in ancient times the voice was more likely to be that of an authority figure such as the father, chief, or king â€” and therefore male. I'll have to see if someone has done a study on the gender of voices heard in hallucinating patients.

Further to my final point: perhaps the fact that the auditory hallucinations have the voices of (often dead) relatives, explains the widespread belief, particularly in ancient times, in the afterlife. Hearing an invisible dead relative speaking to you certainly seems a lot like they are still around but in non-corporeal form! (Seems such an obvious point I wonder why I didn't think of it before.)

Moreover, judging by the quadriplegic cases, these voices sometimes talk about heaven and even pass on messages from (other) dead relatives. Though that much may well of course arise from religious influence (belief in heaven/the afterlife) from before the subject had any hallucinations.

Yes definitely. There is a lot of evidence for this that Jaynes's discusses in The Origin, see for example around p. 143: "The king dead is a living god..." Also the heads found in ancient dwellings at Jericho, etc. (p. 151).

Also in Reflections, the example of the dual burials in ancient China, with the second burial presumably taking place after the voices have stopped (around p. 397).